In process-measurement and automation technology, physical parameters of a fluid flowing in a pipe, such as mass flow rate, density, and/or viscosity, are frequently measured by means of meters which, using a vibratory transducer traversed by the fluid and a measuring and control circuit connected thereto, induce reaction forces in the fluid, such as Coriolis forces corresponding to the mass flow rate and inertial forces corresponding to the density, and derive therefrom a measurement signal representing the respective mass flow rate and/or density of the fluid.
Such mass flowmeters or Coriolis mass flowmeter-densiineters are disclosed, for example, in WO-A 01/33174, WO-A 00/57141, WO-A 98/07009, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,301,557, U.S. Pat. No. 4,876,898, EP-A 553 939, EP-A 1 001 254, EP-A 553 939, EP-A 1 001 254, or European Patent Application EP 01 120 561.4, which was not published prior to the filing data of the present application. To conduct the fluid, each of the transducers comprises at least one flow tube held in a support frame and having a bent tube segment which in operation, driven by an electromechanical excitation assembly, is caused to vibrate in order to produce the above-mentioned reaction forces. To sense vibrations of the tube segment, particularly inlet-side and outlet-side vibrations, the transducers each comprise a sensor arrangement which responds to motions of the tube segment.
The flow tube as well as the excitation assembly and the sensor arrangement are enclosed by a case cap connected with, particularly welded to, the support frame. Besides supporting the at least one flow tube, the case, formed by the support frame and the case cap, serves in particular to protect the flow tube, the excitation assembly, and the sensor arrangement as well as other internal components from external environmental influences, such as dust or splashed water.
Such a case cap for a vibratory transducer for encasing at least one bent segment of a fluid-conducting flow tube, which segment vibrates during operation of the transducer, is disclosed, for example, in WO-A 00/57141, U.S. Pat. No. 5,301,557, EP-A 1 001 254, or European Patent Application EP 01 120 561.4. It comprises:    a channel-shaped first cap segment            with a first segment edge and        with a second segment edge essentially identical in shape to the first segment edge;            an essentially plane second cap segment            which is connected via a first segment edge with the first segment edge of the first cap segment; and            a third cap segment, which is essentially mirror-symmetric with respect to the second cap segment and is connected via a first segment edge with the second segment edge of the first cap segment.
A requirement frequently placed by users on such transducer cases, particularly on the respective case caps, is that if the tube segment bursts, they can withstand the internal pressure, which is then generally well above the external pressure, without leaking for at least a predetermined time, see also WO-A 00/57141, U.S. Pat. No. 6,044,715, U.S. Pat. No. 5,301,557, or EP-A 1 001 254. At least for applications involving toxic or highly inflammable fluids, the case may also have to meet the requirements placed on a safety vessel.
While the required mechanical strength, particularly the required pressure resistance, can generally be readily provided by the support frame, particularly if the latter is in the form of a cylindrical tube, additional measures to increase the pressure resistance must be taken on the case caps because their three-dimensional shape is unfavorable for safety vessels.
The caps of such cases are generally integrally formed as deep-drawn semifinished products. They may also be composed of individual shell-shaped semifinished products which, as proposed in European Patent Application EP 01 120 561.4, for example, may be of small wall thickness, namely much less than 5 mm, or, as indicated in U.S. Pat. No. 6,044,715, for example, of greater wall thickness. While, as also stated in European Patent Application EP 01 120 561.4, case caps fabricated by deep drawing can be made economically only in large quantities because deep-drawing dies are very expensive, the composite case caps, which can be made at lower cost even in small numbers often do not have the required pressure resistances.